The present invention relates to a non-invasive measurement of blood constituents which is adapted to a measurement of oxygen saturation in blood in a living body, etc., and more specifically, relates to a measurement of venous blood constituents in a living body.
Among known non-invasive measuring apparatuses of blood constituents is an apparatus which optically measures those through the application of the Lambert-Beer law. FIG. 1 shows such an apparatus to measure the oxygen saturation, i.e., an existence ratio of oxygenated hemoglobin.
In this figure, light beams emitted from the light source 31 is transmitted through a measuring part, e.g., a finger f, of a living body and are received by photodetectors 33.sub.1 and 33.sub.2. The light beams with wavelengths of .lambda..sub.1 (e.g., 805 nm) and .lambda..sub.2 (e.g., 650 nm) are selected by interference filters 32.sub.1 and 32.sub.2, respectively. That is, the light quantities of transmission light beams with wavelength of .lambda..sub.1 and .lambda..sub.2 are selectively detected.
Output signals from the photodetectors 33.sub.1 and 33.sub.2 are processed by respective signal processing circuits 39.sub.1 and 39.sub.2 and provided to a multiplexer (MUX) 40. The signal processing circuits 39.sub.1 comprises a logarithmic amplifier 34, a high-pass filter (HPF) 35, an amplifier 36, a low-pass filter (LPF) 37, and a sample hold (S/H) circuit 38. Another signal processing circuit 39.sub.2 has the same constition as the signal processing circuit 39.sub.1.
A received light quantity I of the photodetector is in accordance with the Lambert-Beer law and expressed as: EQU I=I.sub.0 F.sub.T.10.sup.-p.10.sup.-q ( 1) EQU where EQU p=.alpha.'.gamma.d, q=.alpha..gamma.l.
In the above equation, F.sub.T denotes an absorption degree by living body tissues; .alpha. and .alpha., light absorption coefficients of venous blood and arterial blood, respectively; .gamma., a blood density; d and l, widths of venous and arterial blood, respectively. The photodetected signal E (voltage value) from the photodetector 33.sub.1 is expressed as: EQU E=AI.sub.0 F.sub.T.10.sup.-p.10.sup.-q ( 2)
where A denotes a gain of photoelectric conversion including the sensitivity of the photodetector 33.sub.1.
The logarithmic conversion of the photodetected signal E from the photodetector 33.sub.1 results in: EQU log E=log AI.sub.0 F.sub.T -p-q=log AI.sub.0 F.sub.T -.alpha.'.gamma.d-.alpha..gamma.l. (3)
The terms log AI.sub.0 F.sub.T and -.alpha.'.gamma.d are signal components of the tissues and venous blood, respectively, and do not vary with time. On the other hand, the term -.alpha..gamma.l is a signal component of the arterial blood and therefore varies with time in synchronism with a heartbeat as a ripple signal shown in FIG. 2.
The time-variant signal component of the arterial blood is separated from the other components by the high-pass filter 35. The separated signal component of the arterial blood is amplified by the amplifier 36, filtered by the low-pass filter 37 so as to eliminate hum components originating from a power supply, and held by the sample hold circuit 38. The signal processing circuit 39.sub.2 performs the same processing as described above, so that the arterial signal component which corresponds to .lambda..sub.2 is held by the sample hold circuit therein.
The respective arterial signal components corresponding to .lambda..sub.1 and .lambda..sub.2 are successively provided to an A/D converter 41 through the multiplexer 40, converted to digital signals, and received by a CPU 42. The CPU 42 calculates the ratio Y of the component -.alpha..sub.1 .gamma.l corresponding to .lambda..sub.1 to the component -.alpha..sub.2 .gamma.l corresponding to .lambda..sub.2 : ##EQU1## The ratio Y can be calculated easily by using an amplitude ratio or waveform area ratio of the arterial signal components.
The CPU 42 further calculates the oxygen saturation S.sub.a O.sub.2 on the basis of the following equation (5): EQU S.sub.a O.sub.2 =B-CY (5)
where B and C are constants related to absorption coefficients of deoxyhemoglobin and oxyhemoglobin, respectively. The calculated S.sub.a O.sub.2 is displayed on a display unit 43.
It is understood that if the oxygen saturation of arterial blood and that of venous blood could be compared, the information concerning the activity of tissues, etc., which is very useful, would be obtained. However, the above-described apparatus cannot measure the oxygen saturation and other blood constituent characteristics (e.g., density) of venous blood.
This is because, for example, there exists hemoglobin in the venous blood as well as in the arterial blood and, as described above, the venous signal component, -.alpha.'.gamma.d, is time-invariant, so that the venous signal component cannot be separated from the signal components of arterial blood and tissues.